My understanding is that the insurance program covering the buildings had a $3.5B per occurrence limit which would have been sufficient except most of the insurance carriers successfully argued in court that even though it was two separate planes, it was one singular terrorist attack and thus one occurrence.
The attack also occurred a day or two after the insurance policies were renewed, meaning the full policies had not been finalized yet and there was only an insurance binder on the buildings which is a lot less enforceable than a full policy.
In essence, if the intent was insurance fraud, it was horribly executed
